The healthy intestinal tract is characterized by continuous, well-controlled inflammation. This is caused by an interaction of the enteric epithelium normal bacterial flora that are usually in a state of homeostasis. Once this balance is broken, the immune system becomes chronically activated with overproduction of inflammatory cytokines, leading to the development of inflammatory bowel disease. The overall goal of this project is to define the molecular mechanisms by which the intestinal innate immune system maintains homeostasis with commensal or pathogenic bacterial flora in the gut. Recent findings of a family of proteins belonging to Nod or NBD-LRR proteins shed a new light on our understanding of bacterial recognition. One Nod protein, Nod2 detects moieties of the bacterial cell wall and its mutation is associated with Crohn's disease via unknown mechanisms. A copy of the mutated alleles of Nod2 increases the risk for Crohn's disease by ~2-4 fold whereas homozygosity or compound heterozygosity of Nod2 mutations increases the risk ~40 fold, suggesting mutations in Nod2 act in a recessive fashion for Crohn's disease. We have recently shown that Nod2 activates innate and adaptive immune responses through recognition of MDP (muramyl dipetide), a moiety of bacterial peptidoglycan, and protects the intestine from bacterial infection by regulating expression of a subgroup of intestinal anti-microbial peptides. Therefore, we hypothesize that Nod2 maintains intestinal homeostasis by controlling commensal and pathogenic bacteria via detection of bacteria or bacterial components and optimum production of anti-bacterial peptides in the intestine. These hypotheses will be evaluated by studies addressing the following specific aims. Aim 1 proposes to define the Nod2-mediated signaling pathways leading to the expression of anti-microbial peptides in the intestine. Aim 2 proposes to delineate the effect of Nod2 on intestinal homeostasis by using mouse colitis models. Aim 3 proposes to determine the cell types involved in Nod2-dependent intestinal pathology. These studies should provide novel insights into the mechanism of intestinal homeostasis and the physiological functions of Nod2, which is of great significance to the pathogenesis of inflammatory bowel disease.